Deductions & credits

So in the image above, Bob would only add a subtraction of  $300 in the Community Property Subtraction Adjustment field right and leave the other field blank?

And in the second image, I would do the same thing with tax (most likely a subtraction adjustment for Bob) withholding right? Another issue I noticed is that when I enter a positive value in the Tax Withholding - Subtraction Adjustment federal, state, and local amount fields, it says "maximum value is 0". I was wondering why this happens. Am i supposed to put a negative value? e.g. -300